changes system shutdown order: "user" actor first

[StefanBertels]

I had this small issue on shutdownAll: If an exception is thrown in a "user" actor during shutdown then this error caused an error entry on ProcessSystemLog saying "/root/system/errors" is missing -- because the "system" actor already was shut down.

This PR should fix this by shutting down "/root/user" first and everything else (including "/root/system") after that.

[louthy]

@StefanBertels I'm going to accept this, but it's really an inefficient way of getting the result you want. We could easily have a bit of code that does:

    shutdown(User);
    shutdown(System);

Rather than forcing a sort on every node.

[StefanBertels]

Thanks for merging.

Yes, I decided to use somewhat ugly and potentially inefficient sorting instead of explicit shutdown calls.

But I don't see the issue in your last comment.

I didn't want to break any current or future code and therefore I refactored in a way that always works -- regardless of the number of children of the root actor (and regardless of their name) => robustness.

I don't see a practical performance issue, neither, because there is not "sort on every node". This is ActorSystem.Dispose() which AFAIK means this is just called once (root node). Additionally there is a very limited number of items to sort at all (probably fixed to two in the current implementation). ;-)